Fluorinated cation exchange membrane and process for preparing the same

ABSTRACT

A fluorinated cation exchange membrane characterized by the gradual decrease in proportion of the density of carboxylic acid and/or carboxylate groups relative to the total density of carboxylic acid and/or carboxylate groups and sulfonic acid and/or sulfonate groups from one surface of the membrane to the other surface or an internal plane therewithin. The cation exchange membrane of the present invention can be prepared by treating one surface of a membrane comprising a fluorocarbon polymer containing pendant groups of the formula: 
     
         --OCF.sub.2 CF.sub.2 SO.sub.2 X 
    
     wherein each X independently is fluorine, chlorine, bromine, hydrogen, ammonium, a quaternary ammonium or a metal atom, with a treating agent having a reducing activity in the presence of a reaction controlling agent selected from carboxylic acids, sulfonic acids, alcohols, nitriles and ethers. The cation exchange membrane of the present invention has an excellent performance in use for electrolysis and can be used stably under severe electrolysis conditions for a long period of time without bringing about partial cleavage or peeling-off, cracking and/or blistering of the carboxylic acid group-richer surface layer thereof.

This application is a divisional of copending application Ser. No.152,780, filed on May 23, 1980, and now U.S. Pat. No. 4,332,665.

This invention relates to an improved cation exchange membrane of afluorinated polymer and a method for the production thereof. Moreparticularly, this invention relates to a cation exchange membrane of apolyfluorocarbon polymer containing pendant carboxylic acid and/orcarboxylate (salt of carboxylic acid) groups and pendant sulfonic acidand/or sulfonate (salt of sulfonic acid) groups which is so improved asnot to bring about partial cleavage or peeling-off, dot-like swellingand/or blistering of the carboxylic acid group-richer surface (richerwhen compared with the other surface) layer of the membrane in thecourse of the electrolysis of, for example, an aqueous solution of analkali metal halide by the use of the cation exchange membrane undersevere conditions and which is characterized by the gradual decrease inproportion of the density of carboxylic acid and/or carboxylate groupsrelative to the total density of carboxylic acid and/or carboxylategroups and sulfonic acid and/or sulfonate groups from one surface of themembrane to the other surface or an internal plane therewithin, and aprocess for producing such a membrane.

Many cation exchange membranes of fluorinated polymers have beenproposed for use in the electrolysis of an aqueous solution of an alkalimetal halide. For example, a cation exchange membrane of a fluorinatedpolymer containing pendant sulfonic acid and/or sulfonate groups hasbeen known which is obtained by saponification (hydrolysis) of amembrane prepared from a copolymer of tetrafluoroethylene andperfluoro-3,6-dioxa-4-methyl-7-octene-sulfonyl fluoride. This knownfluorinated polymer type cation exchange membrane containing onlysulfonic acid and/or sulfonate groups, however, has such a disadvantagethat the membrane, when used in the electrolysis of an aqueous solutionof an alkali metal halide, tends to permit penetration therethrough ofhydroxyl ions back-migrating from the cathode compartment because themembrane is liable to swell owing to the highly hydrophilic nature ofthe sulfonic acid and/or sulfonate groups. This disadvantage leads tolow current efficiency during the electrolysis. Particularly when theelectrolysis is carried out in the production of an aqueous solution ofcaustic soda having a sodium hydroxide concentration of at least 20% byweight, the current efficiency is so extremely low that the process iseconomically disadvantageous as compared with the electrolysis ofaqueous solutions of sodium chloride by the conventional mercury processor diaphragm process.

The drawback of such low current efficiency can be alleviated to someextent by lowering the exchange capacity of a membrane having sulfonicacid and/or sulfonate groups as ion exchange groups, for example, toless than 0.7 milliequivalent per gram of dry resin in the hydride state(H or acid form) constituting the membrane because the hydrophilicnature of the membrane is so lowered that the swelling of the membraneduring electrolysis is suppressed to some extent and, hence, theconcentration of fixed ions in the membrane, which concentration is adetermining factor in current efficiency, is rather high as comparedwith that in a swollen membrane made of a sulfonic acid and/or sulfonategroup-containing resin having a higher exchange capacity in a dry state.For example, the use of a membrane having a lowered exchange capacityimproves current efficiency in the electrolysis of sodium chloride forobtaining a 20% by weight aqueous solution of caustic soda to a level ofabout 80%. However, the lowering of exchange capacity of the membranefor improving the current efficiency results in such a serious increasein electric resistance of the membrane that the electrolysis processcannot be practiced economically. However low the exchange capacity of aperfluorinated cation exchange membrane containing only sulfonic acidand/or sulfonate groups as ion exchange groups may be with such asacrifice that the electric resistance of it may increase, it is quitedifficult to attain a current efficiency of about 90% duringelectrolysis by using such a membrane.

On the other hand, cation exchange membranes of fluorocarbon polymerscontaining only carboxylic acid and/or carboxylate groups as ionexchange groups are disclosed in British Pat. No. 1,497,748 and U.S.Pat. No. 4,065,366. Such membranes show a lesser tendency to swell and,hence, can have such a high concentration of fixed ions therein duringelectrolysis that a current efficiency of 90% or more can be realized.Further, the membranes are chemically stable enough to be used inelectrolysis under usual conditions. However, the membranes containingcarboxylic acid and/or carboxylate groups have higher electricresistances than the aforementioned membranes containing sulfonic acidand/or sulfonate groups. Therefore, when a membrane containing onlycarboxylic acid and/or carboxylate groups is used in electrolysis at ahigh current density, there arises a drawback that the unit consumptionof electric power is extremely increased. Further, when such a membraneis used in an aqueous alkali solution having a high alkali concentrationunder severe conditions for a long period of time, there arises adrawback that the membrane gradually shrinks and becomes so brittle thatlaminar cleavage or peeling off and/or cracking of the surface portionof the membrane occurs, leading to the lowering of current efficiency.

In order to obviate the drawbacks accompanying the use of the membranecontaining only carboxylic acid and/or carboxylate groups as ionexchange groups, there have been proposed a cation exchange membranewhich is obtained by blending a fluorocarbon polymer containingcarboxylic acid and/or carboxylate groups or groups changeable tocarboxylic acid groups (hereinafter often referred to as "precursorgroups") and a fluorocarbon polymer containing sulfonic acid and/orsulfonate groups or groups changeable to sulfonic acid groups(hereinafter often referred to as "precursor groups") and forming theblend into a membrane and, if the precursor groups are present in themembrane, saponifying the membrane (Japanese Patent ApplicationLaid-Open Specification No. 36,589/1977) and a cation exchange membranewhich is obtained by laminating a membrane of a fluorocarbon polymercontaining carboxylic acid and/or carboxylate groups or precursor groupsthereof and a membrane of a fluorocarbon polymer containing sulfonicacid and/or sulfonate groups or precursor groups thereof and, if theprecursor groups are present, saponifying the laminated membrane (GermanOffenlegungsschrift No. 2,817,344). However, these polymers are so poorin compatibility with each other that it may be difficult to achievesatisfactory blending or lamination. Therefore, there remains anunsolved problem that the cation exchange membranes prepared from thosepolymers are liable to bring about peeling-off or cracking and/orblistering during the use thereof in electrolysis under severeconditions. Further, the cation exchange membrane of those polymersblended with each other is quite unsatisfactory from the viewpoint ofskillful utilization of the high current efficiency-providing effect ofthe carboxylic acid and/or carboxylate groups and the low electricresistance-giving effect of the sulfonic acid and/or sulfonate groupsbecause the membrane only shows a performance lying between that of amembrane containing only carboxylic acid and/or carboxylate groups andthat of a membrane containing only sulfonic acid and/or sulfonategroups. This can also be said with respect to cation exchange membranesobtained by terpolymerizing a monomer having a carboxylic acid and/orcarboxylate group or a precursor group thereof, a monomer having asulfonic acid and/or sulfonate group or a precursor group thereof and afluorinated olefin and forming the terpolymer into a membrane and, ifthe precursor groups are present in the membrane, saponifying themembrane (the above-mentioned Japanese Patent Application Laid-OpenSpecification No. 36,589/1977 and Japanese Patent Application Laid-OpenSpecification No. 23,192/1977).

In U.S. Pat. No. 4,151,053 (corresponding to British Pat. No. 1,523,047)and German Offenlegungsschrift No. 2,817,315, there are disclosed cationexchange membranes obtained by subjecting to chemical treatment onesurface layer portion of a membrane of a fluorocarbon polymer containingsulfonic acid and/or sulfonate groups to form carboxylic acid and/orcarboxylate groups in the one surface layer portion. Such cationexchange membranes are so effective for substantial prevention ofback-migration or diffusion therethrough of hydroxide ions duringelectrolysis that a high current efficiency can be achieved. Further,since such cation exchange membranes, when positioned in an electrolyticcell, have the carboxylic acid and/or carboxylate groups only in thevery thin surface layer portions thereof on the side facing a cathodeand the sulfonic acid and/or sulfonate groups having a highlyhydrophilic nature in the remaining portions thereof, the electricresistances of the membranes are so low that the electrolysis processcan be practiced with a great advantage from the viewpoint of unitconsumption of electric power.

However, there is a serious demand in the art for the development of acation exchange membrane capable of being used under severerelectrolysis conditions, i.e. at a higher current density and at ahigher temperature. In this sense, the above-mentioned conventionalcation exchange membranes are still unsatisfactory because unfavorablephenomena such as partial cleavage or peeling-off, dot-like swellingand/or blistering of the surface layer portion of the membranes occurduring electrolysis under such severer conditions as is apparent fromComparative Example 1 which will be given later.

It is therefore an object of the present invention to provide a cationexchange membrane which can be used in electrolysis under severeconditions without the occurrence of unfavorable phenomena such aspartial cleavage or peeling-off, dot-like swelling and/or blistering ofthe surface layer portion of the membrane and which has a highperformance in use for electrolysis. Another object of the presentinvention is to provide a process for preparing a cation exchangemembrane of the kind described above.

The foregoing and other objects, features and advantages of the presentinvention will be apparent to those skilled in the art from thefollowing detailed description and appended claims taken in connectionwith the accompanying drawing in which:

The FIGURE is a graph showing the relationship between the proportion(p) and the thickness (t), both of which are as defined hereinafter, inthe cation exchange resin of the present invention prepared in Example 1which will be given later.

We have made investigations into the causes of such unfavorablephenomena as mentioned above and intensive researches with a view todeveloping a cation exchange membrane which does not cause theunfavorable phenomena during electrolysis even under severe conditionsmentioned above. As a result, we have found that the unfavorablephenomena as mentioned above do not occur in a cation exchange membranewherein the proportion of the density of carboxylic acid and/orcarboxylate groups relative to the total density of carboxylic acidand/or carboxylate groups and sulfonic acid and/or sulfonate groups insurfaces of the membrane or cross-sectional planes parallel to thesurfaces of the membrane gradually decreases from one surface of themembrane to the other surface or a certain internal cross-sectionthereof.

More specifically, in accordance with one aspect of the presentinvention, there is provided a fluorinated cation exchange membranecomprising a fluorocarbon polymer containing pendant carboxylic acidand/or carboxylate groups and pendant sulfonic acid and/or sulfonategroups, the porportion of the density of pendant carboxylic acid and/orcarboxylate groups relative to the total density of pendant carboxylicacid and/or carboxylate groups and pendant sulfonic acid and/orsulfonate groups being different between one surface and an internalplane in cross-section parallel to the surfaces of the membrane; whichmembrane comprises a fluorocarbon polymer containing pendant groups ofthe formula (1):

    --OCF.sub.2 COOM                                           (1)

wherein each M independently is hydrogen, ammonium, a quaternaryammonium or a metal atom, and pendant groups of the formula (2):

    --OCF.sub.2 CF.sub.2 SO.sub.3 M                            (2)

wherein each M independently is as defined above, and wherein theproportion (p) represented by the equation (a):

    p=A/(A+B)×100 (%)                                    (a)

wherein A is the density of pendant groups of the formula (1) and B isthe density of pendant groups of the formula (2), is at least 20% in onesurface of the membrane, and said proportion (p) gradually decreasesfrom the one surface to the other surface or that plane within themembrane where A reaches zero, said proportion (p) and a thickness (t)in microns between the one surface and the other surface or a planewithin the membrane in cross-section parallel to the surfaces of themembrane always satisfying the following inequality (b):

    {Δp/Δt|≦12 (%/μ)            (b).

The inequality (b) means that the maximum descending gradient(|Δp/Δt|max) of the proportion (p) is at most 12% per micron inthickness of the membrane in a graph of Cartesian coordinates systemhaving an ordinate representing the proportion (p) (%) and an abscissarepresenting the thickness (t) (μ).

The fluorocarbon polymer constituting the cation exchange membrane ofthe present invention may further contain other ion exchange groupsselected from phosphoric acid groups (--PO₃ H₂), phosphate groups (saltof phosphoric acid group), phosphorous acid groups (--PO₂ H₂), phosphitegroups (salt of phosphorous acid group), phenolic hydroxyl groups,phenolic hydroxylate groups (salt of phenolic hydroxyl group) andsulfonamide groups (--SO₂ NH₂, --SO₂ NHR wherein R is alkyl, or --SO₂NHR'NH₂ wherein R' is alkylene: reference may be made to U.S. Pat. Nos.3,969,285 and 4,085,071).

The cation exchange membrane of the present invention is characterizedin that it has such an excellent performance in use for electrolysisthat it gives high current efficiency and has low electric resistance,that it is so stable, with respect to cleavage or peeling-off, dot-likeswelling and/or blistering of the surface layer portion of the membrane,under severer electrolysis conditions than the usual electrolysisconditions as compared with conventional cation exchange membranes thatthe excellent performance of the membrane in use for electrolysis can bemaintained for a long period of time, and that the production of themembrane is easy and inexpensive.

The excellent performance of the cation exchange membrane of the presentinvention in use for electrolysis can be attributed to the structure ofthe membrane that the proportion (p) is 20% to 100%, preferably 40% to100%, more preferably 60% to 100%, in one surface of the membrane, theproportion (p) gradually decreasing from the one surface to the othersurface or that plane within the membrane where A reaches zero, and themaximum descending gradient (|Δp/Δt|max) of proportion (p) to thickness(t) is at most 12%/μ, preferably in the range of 0.1 to 12%/μ, morepreferably in the range of 0.5 to 10%/μ, most preferably in the range of2 to 8%/μ. The proportion (p) in the surface opposite to said onesurface of the membrane is preferably 0%, that is, the membrane of thepresent invention preferably contains substantially no carboxylic acidand/or carboxylate groups but sulfonic acid and/or sulfonate groups inthe surface opposite to said one surface of the membrane.

When the cation exchange membrane of the present invention is used inthe electrolysis of an aqueous solution of an alkali metal halide, it isusually advantageous to position the membrane in an electrolytic cell sothat the carboxylic acid and/or carboxylate group-richer surface of themembrane faces the cathode. In this case, said surface shrinks becauseof the presence of carboxylic acid and/or carboxylate groups in thesurface when contacted with a high concentration aqueous solution of analkali. As a result, the concentration of fixed ions in the surfacebecomes so high that the membrane impedes effectively the penetration orback-migration therethrough and diffusion therein of hydroxyl ions,leading to high current efficiency.

The optimum proportion (p) in the carboxylic acid and/or carboxylategroup-richer surface of the membrane of the present invention may bechosen depending on the equivalent weight (EW) of the membrane or thelayer thereof containing carboxylic acid and/or carboxylate groups, andvarious factors such as a current density, electrolysis temperature andalkali concentration when the membrane is used in the electrolysis of anaqueous solution of an alkali metal halide. Equivalent weight is theweight of dry polymer in grams which contains one equivalent ofpotential ion exchange capacity. In general, as the equivalent weight ofthe membrane or the layer thereof containing carboxylic acid and/orcarboxylate groups is higher, the proportion (p) may be lower. Theequivalent weight of the membrane or the layer thereof containingcarboxylic acid and/or carboxylate groups may be 1,000 to 2,800,preferably 1,100 to 2,000, more preferably 1,100 to 1,700. When theequivalent weight exceeds 2,800, the electric resistance of a membranetends to become disadvantageously high. When the equivalent weight isless than 1,000, the mechanical strength of a membrane in use forelectrolysis tends to be insufficient.

According to the preferred embodiment of the present invention, a thecation exchange membrane contains carboxylic acid and/or carboxylategroups mainly in a thin layer on the side of one surface of the membraneand substantially contains only sulfonic acid and/or sulfonate groups inthe remainder constituting the majority of the membrane. In this case,when alkali metal ions migrate from an anode chamber to a cathodechamber, the electric resistance of the membrane is extremely low ascompared with a membrane containing only carboxylic acid and/orcarboxylate groups as ion exchange groups.

The above-mentioned cation exchange membrane may be made of a compositemembrane composed of two kinds of stratums of fluorinated polymers whichdiffer in equivalent weight by 150 or more. In this case, it ispreferred that the thickness of the stratum of a fluorinated polymerhaving the higher equivalent weight be up to a half the thickness of thewhole membrane, and that carboxylic acid and/or carboxylate groups bepresent in the outer surface portion of the stratum of the fluorinatedpolymer having the higher equivalent weight.

The thickness of the cation exchange membrane of the present inventionis generally in the range of from 40μ to 500μ, preferably in the rangeof 100μ to 250μ. The thickness of the layer containing carboxylic acidand/or carboxylate groups may be chosen depending on the equivalentweight of the layer and the electrolysis conditions under which themembrane of the present invention is used. In general, however, thethickness of a layer within the membrane from the carboxylic acid and/orcarboxylate group-containing surface to that plane within the membranewhere the density of carboxylic acid and/or carboxylate groups reacheszero may be at least 2.5μ, preferably at least 7.5μ, and the upper limitof the above-mentioned thickness varies depending on the permissibleelectric resistance of the membrane.

The cation exchange membrane of the present invention is very stable ascompared with conventional cation exchange membranes even when it isused under such electrolysis conditions that the membrane is in contactwith an aqueous high concentration alkali solution at a high currentdensity and at a high temperature. The reason for this resides in thatthe proportion (p) of the density of carboxylic acid and/or carboxylategroups relative to the total density of carboxylic acid and/orcarboxylate groups and sulfonic acid and/or sulfonate groups graduallydecreases with the gradient being within a specific range from onesurface of the membrane to the other surface or that plane within themembrane where the density of carboxylic acid and/or carboxylate groupsreaches zero.

Cation exchange membranes as disclosed in Japanese Patent ApplicationLaid-Open Specification No. 36,589/1977 and German OffenlegungsschriftNo. 2,817,344 which are prepared by a blending method or a laminationmethod wherein a carboxylic acid and/or carboxylate group-containingpolymer and a sulfonic acid and/or sulfonate group-containing polymerare used, are liable, owing to insufficient blending or lamination, tobring about peeling-off (particularly in the case of a laminatedmembrane) or cracking and/or blistering in a short period of time whenused in electrolysis under severe conditions, as described hereinbefore.

According to our knowledge, cation exchange membranes as disclosed inU.S. Pat. No. 4,151,053 and German Offenlegungsschrift No. 2,817,315which are prepared by chemical treatment to convert sulfonic acid groupsin one surface layer of a membrane into carboxylic acid groups, are alsoliable to bring about partial peeling-off, dot-like swelling and/orblistering of the carboxylic acid and/or carboxylate group-containinglayers thereof in electrolysis under severe conditions because themembranes have substantially no gradually decreasing density ofcarboxylic acid and/or carboxylate groups with gradients as specified inthe present application from one surface of each membrane to the othersurface or that plane within each membrane where the density ofcarboxylic acid and/or carboxylate groups reaches zero. Accordingly,such membranes are unable to prevent the lowering of current efficiencyand the rise of electric resistance.

By contrast, the membrane of the present invention can be used even at ahigh current density of, for example, 70 A/dm² for a long period of timewithout the occurrence of peeling-off, cracking etc. of the carboxylicacid and/or carboxylate group-containing layer of the membrane. Thus,the high performance of the membrane in use for electrolysis can bestably maintained for a long period of time.

The membrane of the present invention may be laminated to reinforcingmaterials to improve the mechanical strength. For this purpose, fabricsor nets made of polytetrafluoroethylene fibers are most suitable. Aporous polytetrafluoroethylene sheet or film is also useful. Areinforcing material is usually embedded in the sulfonic acid and/orsulfonate group-richer surface portion of the membrane. Where themembrane is a composite membrane as described hereinbefore, thereinforcing material is usually embedded in the stratu of a polymerhaving a lower equivalent weight. Alternatively, a fibrouspolytetrafluoroethylene may be incorporated into the membrane forimproving the mechanical strength thereof.

In accordance with another aspect of the present invention, there isprovided a process for the preparation of a fluorinated cation exchangemembrane comprising treating, with a treating agent selected from thegroup consisting of an aqueous reducing solution of an inorganic acid,an aqueous reducing solution of an inorganic salt, an aqueous reducingsolution of hydrazine, an aqueous reducing solution of an inorganic acidand an inorganic salt, and an aqueous reducing solution of an inorganicsalt and hydrazine, one surface of a membrane comprising a fluorocarbonpolymer containing pendant groups of the formula (3):

    --OCF.sub.2 CF.sub.2 SO.sub.2 X                            (3)

wherein each X independently is fluorine, chlorine, bromine, hydrogen,ammonium, a quaternary ammonium or a metal atom, to convert part of thependant groups of the formula (3) into pendant groups of the formula(1):

    --OCF.sub.2 COOM                                           (1)

wherein each M independently is hydrogen, ammonium, a quaternaryammonium or a metal atom, said fluorinated cation exchange membranecomprising a fluorocarbon polymer containing pendant carboxylic acidand/or carboxylate groups and pendant sulfonic acid and/or sulfonategroups, the proportion of the density of pendant carboxylic acid and/orcarboxylate groups relative to the total density of pendant carboxylicacid and/or carboxylate groups and pendant sulfonic acid and/orsulfonate groups being different between one surface and an internalplane in cross-section parallel to the surfaces of the membrane;

which process is characterized in that one surface of a membranecomprising a fluorocarbon polymer containing pendant groups of theformula (3):

    --OCF.sub.2 CF.sub.2 SO.sub.2 X                            (3)

wherein each X independently is fluorine, chlorine, bromine, hydrogen,ammonium, a quaternary ammonium or a metal atom, is treated with atreating agent selected from the group consisting of an aqueous reducingsolution of at least one inorganic acid, an aqueous reducing solution ofat least one inorganic salt, an aqueous reducing solution of hydrazine,an aqueous reducing solution of at least one inorganic acid and at leastone inorganic salt, and an aqueous reducing solution of at least oneinorganic salt and hydrazine, in the presence of at least one reactioncontrolling agent selected from the group consisting of C₁ -C₁₂carboxylic acids, C₁ -C₁₂ sulfonic acids, C₁ -C₁₂ alcohols, C₁ -C₁₂nitriles and C₂ -C₁₂ ethers, provided that when the treating agentincludes hydrazine, said at least one reaction controlling agent isselected from C₁ -C₁₂ alcohols, C₁ -C₁₂ nitriles and C₂ -C₁₂ ethers,thereby to prepare a fluorinated cation exchange membrane

which comprises a fluorocarbon polymer containing pendant groups of theformula (1):

    --OCF.sub.2 COOM                                           (1)

wherein each M independently is hydrogen, ammonium, a quaternaryammonium or a metal atom, and pendant groups of the formula (2):

    --OCF.sub.2 CF.sub.2 SO.sub.3 M                            (2)

wherein each M independently is as defined above, and wherein theproportion (p) represented by the equation (a):

    p=A/(A+B)×100(%)                                     (a)

wherein A is the density of pendant groups of the formula (1) and B isthe density of pendant groups of the formula (2), is at least 20% in onesurface of the membrane, and said proportion (p) gradually decreasesfrom the one surface to the other surface or that plane within themembrane where A reaches zero, said proportion (p) and a thickness (t)in microns between the one surface and the other surface or a planewithin the membrane in cross-section parallel to the surfaces of themembrane always satisfying the following inequality (b):

    |Δp/Δt|≦12(%/μ)    (b).

The membrane comprising a fluorocarbon polymer containing pendant groupsof the formula (3) which membrane is to be used as a starting materialin the process of the present invention can be prepared as follows.

Firstly, the copolymerization of tetrafluoroethylene with at least onefluorinated vinyl monomer containing a sulfonyl fluoride group andrepresented by the general formula (4): ##STR1## wherein R_(F) is F, CF₃or CF₂ OCF₃ and n is an integer of 0 to 3, is carried out. A fluorinatedvinyl monomer of the formula (4) wherein R_(F) is CF₃ and n is 1 ispreferably used. According to need, at least one fluorinated vinylmonomer of the general formula (5):

    CF.sub.2 ═CF--D                                        (5)

wherein D is Cl, CF₃, OCF₃ or OC₃ F₇, may be used as a third monomer inthe above-mentioned copolymerization.

Other monomer(s) may be used as further optional monomers in smallamounts in the copolymerization for incorporating into the final cationexchange membrane other ion exchange groups as mentioned hereinbeforethan carboxylic acid and/or carboxylate groups and sulfonic acid and/orsulfonate groups.

The mixing molar ratio of tetrafluoroethylene and at least onefluorinated vinyl monomer of the formula (4) and, if desired, at leastone fluorinated vinyl monomer of the formula (5) and other monomer(s)may usually be so adjusted that the copolymer to be obtained has monomerunits in amounts such as will satisfy the following inequality:0.04≦m≦0.15, preferably 0.06≦m≦0.13, more preferably 0.07≦m≦0.13##EQU1## This substantially corresponds to the situation that theequivalent weight of the copolymer is usually in the range of from 1,000to 2,800, preferably in the range of from 1,100 to 2,000, morepreferably in the range of from 1,100 to 1,700.

On the other hand, the molar ratio represented by the formula: ##EQU2##is usually in the range of from 0 to 0.20, preferably in the range offrom 0 to 0.10, more preferably in the range of from 0 to 0.05.

The copolymer used for the production of a membrane to be used as astarting material in the process of the present invention may beprepared according to any of the customary polymerization methods knownin the art for homopolymerization of copolymerization of a fluorinatedethylene, such as methods using a non-aqueous system, methods using anaqueous system and a method using ultraviolet rays. The copolymerizationis usually effected at a temperature of 0° to 200° C. under a pressureof 1 to 200 Kg/cm². The copolymerization in a non-aqueous system iscarried out in an inert fluorinated solvent in mos: cases. As suitableinert fluorinated solvents, there can be mentioned1,1,2-trichloro-1,2,2-trifluoroethane, and perfluorocarbons such asperfluoromethylcyclohexane, perfluorodimethylcyclobutane,perfluorooctanes, perfluorobenzene and the like. The aqueous systempolymerization is accomplished by contacting monomers with an aqueousmedium containing a free radical polymerization initiator and asuspension-forming agent to produce a slurry of polymer particles orgranules, or by contacting monomers with an aqueous medium containing afree radical polymerization initiator and a dispersant inactive for thetelomerization of monomers to produce a colloidal dispersion of polymerparticles, followed by the coagulation of the dispersion.

Secondly, after the polymerization, the resultant copolymer is moltenand shaped into a thin membrane using any of a variety of well knowntechniques.

The copolymer, after being shaped into a membrane, can be laminated witha reinforcing material such as a fabric or net for improvement of themechanical strength. As the reinforcing material, fabrics and nets madeof polytetrafluoroethylene fibers are most suitable. A porouspolytetrafluoroethylene sheet or film is also useful. Alternatively, afibrous polytetrafluoroethylene may be incorporated into the copolymerand the resulting mixture may be molten and shaped into a membrane forimproving the mechanical strength thereof.

In the case of the cation exchange membrane of a composite membranewhich is the preferred embodiment of the invention as described above,two kinds of copolymers differing in equivalent weight by 150 or moreare prepared according to the polymerization methods as described above,followed by shaping into thin films, and fabricating into a compositemembrane. In this composite membrane, it is preferred that the thicknessof the stratum made of the thin film of the polymer having a higherequivalent weight be up to half the thickness of the whole membrane, andthat a reinforcing material as mentioned above, if used, be embedded inthe stratum made of the thin film having a lower equivalent weight.

Thirdly, according to need, groups contained in the copolymerconstituting the membrane and represented by the formula (6):

    --OCF.sub.2 CF.sub.2 SO.sub.2 F                            (6)

are partially or totally converted into groups of the formula (7):

    --OCF.sub.2 CF.sub.2 SO.sub.2 X'                           (7)

wherein each X' independently is chlorine, bromine, hydrogen, ammonium,a quaternary ammonium or a metal atom. The conversion may be effectedaccording to either a method (I) or a method (II) as described below.

METHOD (I)

Sulfonyl fluoride groups contained in the groups of the formula (6) areoptionally saponified to form sulfonic acid and/or sulfonate (salt ofsulfonic acid) groups. The sulfonyl fluoride groups or the sulfonic acidand/or sulfonate groups are subjected to treatment with a reducing agentto form sulfinic acid and/or sulfinate (salt of sulfinic acid) groups.Examples of reducing agents useful in this treatment are metallichydrides of the general formula MeLH₄ (wherein Me is an alkali metal, Lis aluminum or boron), or the general formula MeHy (wherein Me is analkali or alkaline earth metal and y is 1 or 2) among those reducingagents as disclosed in U.S. Pat. No. 4,151,053.

As such metallic hydrides, there can be mentioned lithium aluminumhydride, lithium boron hydride, potassium boron hydride, sodium boronhydride, sodium hydride, lithium hydride, potassium hydride, bariumhydride, calcium hydride and the like. The optimum treating conditionsmay be chosen depending on the kind of reducing agent, the kind offunctional groups to be subjected to the treatment, and the like. Ingeneral, the treatment or reaction temperature is in the range of from-50° C. to 250° C., preferably in the range of from 0° C. to 150° C. Thereducing agent is usually used in the form of a solution. As a solventuseful for the preparation of a solution of the reducing agent, therecan be mentioned water; polar organic solvents such as methanol,tetrahydrofuran, diethyleneglycol dimethyl ether, acetonitrile,propionitrile and benzonitrile; nonpolar organic solvents such asn-hexane, benzene and cyclohexane; and mixed solvents thereof. Theamount of the reducing agent to be used is at least the same inequivalents as that amount of functional groups to be subjected to thetreatment which is present in the copolymer constituting the membrane.However, it is preferred that a largely excessive amount of the reducingagent be used. The pressure employed in the treatment is not criticaland may be atmospheric or super-atmospheric pressures, but the treatmentis usually carried out under atmospheric pressure. The treatment orreaction period of time is usually in the range of from 1 minute to 100hours. The sulfinic acid and/or sulfinate groups formed by the treatmentmay be partially or totally reacted with chlorine and/or bromine to formsulfonyl chloride and/or sulfonyl bromide groups.

METHOD (II)

Sulfonyl fluoride groups contained in the groups of the formula (6) aresaponified to form sulfonic acid and/or sulfonate (salt of sulfonicacid) groups. The sulfonic acid and/or sulfonate groups are reacted witha reactant selected from phosphorus halides and sulfur halides whereinthe halide is chloride or bromide to form sulfonyl chloride and/orsulfonyl bromide groups. Examples of such a reactant include PCl₅,POCl₃, SO₂ Cl₂, PCl₃, PBr₅, POBr₃, PBr₃ and mixtures thereof. Otherhalogenating agents similar to those mentioned above may also be used.In the case of this method (II), only chlorine and bromine are possibleas X' in the formula (7).

Thus, a membrane comprising a fluorocarbon polymer containing pendantgroups of the formula (3):

    --OCF.sub.2 CF.sub.2 SO.sub.2 X                            (3)

wherein each X independently is fluorine, chlorine, bromine, hydrogen,ammonium, a quaternary ammonium and a metal atom, is obtained.

According to the process of the present invention, one surface of amembrane comprising a fluorocarbon polymer containing pendant groups ofthe formula (3) is treated with a treating agent selected from the groupconsisting of an aqueous reducing solution of at least one inorganicacid, an aqueous reducing solution of at least one inorganic salt, anaqueous reducing solution of hydrazine, an aqueous reducing solution ofat least one inorganic acid and at least one inorganic salt, and anaqueous reducing solution of at least one inorganic salt and hydrazine,in the presence of at least one reaction controlling agent selected fromthe group consisting of C₁ -C₁₂ carboxylic acids, C₁ -C₁₂ sulfonicacids, C₁ -C₁₂ alcohols, C₁ -C₁₂ nitriles and C₂ -C₁₂ ethers, providedthat when the treating agent includes hydrazine, said at least onereaction controlling agent is selected from C₁ -C₁₂ alcohols, C₁ -C₁₂nitriles and C₂ -C₁₂ ethers. It is preferred that the treating agentselected from the above-mentioned aqueous reducing solutions be used inthe form of a mixture thereof with a reaction controlling agent. The useof the mixture (solution) of the treating agent and the reactioncontrolling agent is advantageous from the viewpoints of ease inoperation of the treatment and ease in control of the reaction, whichare relatively poor when the treatment is carried out by contacting onesurface of the membrane with the reaction controlling agent andsubsequently contacting the one surface with the treating agent.

Examples of inorganic acids that may be used as a reducing agent in thetreating agent to be used in the process of the present inventioninclude hydriodic acid, hydrobromic acid, hypophosphorous acid, hydrogensulfide, arsenious acid, phosphorous acid, sulfurous acid and nitrousacid. Examples of inorganic salts that may be used as a reducing agentin the treating agent are ammonium, or alkali or alkaline earth metalsalts of the above-mentioned inorganic acids that are soluble in wateror a mixture of water and a reaction controlling agent. A mixture ofdifferent inorganic acids, a mixture of different inorganic salts, amixture of an inorganic acid and an inorganic salt or any otherconceivable mixture may be used in the treating agent as long as theingredients in the mixture are not reactive with each other.

The treating agent containing a reducing agent as mentioned above isusually used in a large excess.

A wide variety of reaction controlling agents can be used in combinationwith an appropriate treating agent in the process of the presentinvention insofar as they are inert to the groups of the formula (3) andthe reducing inorganic acid or its salt or hydrazine contained in thetreating agent. It is noted that hydrazine reacts with an acid to form asalt.

The mechanism of the action of reaction controlling agent for achievingthe purpose of the present invention has not been elucidated yet, but isbelieved to be such that the reaction controlling agent functions as amedium for increasing the affinity of a membrane being treated for areducing agent selected from inorganic acids, inorganic salts andhydrazine and promoting the penetration of the reducing agent into theinterior of the membrane to enable the reducing agent to react with themembrane not only at the surface layer portion but also in the internalportion of the membrane and, in extreme cases, even at the surface layerportion opposite to the surface contacted with the treating agent.

Examples of C₁ -C₁₂ carboxylic acids and C₁ -C₁₂ sulfonic acids that maybe used as the reaction controlling agent in the process of the presentinvention include monobasic or polybasic acids such as formic acid,acetic acid, propionic acid, n-butyric acid, isobutyric acid, n-valericacid, caproic acids, n-heptanoic acid, caprylic acids, lauric acid,fluoroacetic acid, chloroacetic acid, bromoacetic acid, dichloroaceticacid, malonic acid, glutaric acid, trifluoroacetic acid,perfluoropropionic acid, perfluorobutyric acids, perfluorovaleric acids,perfluorocaproic acids, perfluoro-n-heptanoic acid, perfluorocaprylicacids, perfluoroglutaric acid, trifluoromethanesulfonic acid,perfluoroheptanesulfonic acids, methanesulfonic acid, ethanesulfonicacid, propanesulfonic acids, butanesulfonic acids, pentanesulfonicacids, hexanesulfonic acids and heptanesulfonic acids. Preferred areacetic acid, propionic acid, caprylic acids, trifluoroacetic acid,perfluorocaprylic acids, perfluorobutyric acids andperfluoroheptanesulfonic acids.

Examples of C₁ -C₁₂ alcohols that may be used as the reactioncontrolling agent include monohydric or polyhydric alcohols such asmethanol, ethanol, propanol, ethylene glycol, diethylene glycol,1,4-butanediol, 1,8-octanediol and glycerin. Preferred are methanol andethanol.

Examples of C₁ -C₁₂ nitriles that may be used as the reactioncontrolling agent include acetonitrile, propionitrile and adiponitrile.

Examples of C₂ -C₁₂ ethers that may be used as the reaction controllingagent include diethyl ether, tetrahydrofuran, dioxane,monoethyleneglycol dimethyl ether, diethyleneglycol dimethyl ether,triethyleneglycol dimethyl ether and tetraethylene glycol dimethylether.

The treating agent to be used in the process of the present inventionmay usually be an aqueous solution containing 2 to 95% by weight,preferably 5 to 90% by weight, more preferably 10 to 80% by weight, of areducing agent selected from inorganic acids, inorganic salts andhydrazine. When a mixture of the treating agent and a reactioncontrolling agent is used in the process of the present invention, themixture may usually contain 100 ppm to 80% by weight, preferably 1,000ppm to 80% by weight, of the reaction controlling agent. However, it isnoted that the mixing ratio of water and the reducing agent and, ifpresent, the reaction controlling agent cannot be chosen independentlybut depending on the equivalent weight of a membrane to be treated, thecompatibility of the reducing agent and, if present, the reactioncontrolling agent with water, the intended gradient of proportion (p) tothickness (t), and the intended thickness of a layer containingcarboxylic acid and/or carboxylate groups. In general, the higher thereducing agent concentration of the treating agent or the mixturethereof with the reaction controlling agent, the higher the proportion(p) in the treated surface of the membrane tends to be, and the lowerthe mixing ratio of water to the reaction controlling agent in themixture thereof with the treating agent, the larger the thickness of thetreated layer containing carboxylic acid and/or carboxylate groups.

The treatment of one surface of a membrane with the above-mentionedtreating agent or mixture thereof with the reaction controlling agentmay usually be carried out at a temperature of from 0° C. to 150° C.,preferably from 30° C. to 120° C., for a period of from 1 hour to 70hours, preferably from 3 hours to 50 hours. According to need, fordecreasing the amount of sulfinic acid and/or sulfinate groups that maybe formed in the course of the above-mentioned treatment, the thustreated membrane may be treated with an acidic aqueous solution ofhydrochloric acid, sulfuric acid or the like while heating and subjectedto a saponification treatment with an alkali, and, optionally, subjectedto a oxidation treatment with an aqueous solution of sodium hypochloriteor the like.

Thus, a fluorinated cation exchange membrane of the present invention isobtained which comprises a fluorocarbon polymer containing pendantgroups of the formula (1):

    --OCF.sub.2 COOM                                           (1)

wherein each M independently is hydrogen; ammonium; a quaternaryammonium, particularly a quaternary ammonium having a molecular weightof 500 or less; or a metal atom, particularly an alkali metal such assodium or potassium or an alkaline earth metal, and pendant groups ofthe formula (2):

    --OCF.sub.2 CF.sub.2 SO.sub.3 M                            (2)

wherein each M independently is as defined above, and wherein theproportion (p) represented by the equation (a):

    p=A/(A+B)×100 (%)                                    (a)

wherein A is the density of pendant groups of the formula (1) and B isthe density of pendant groups of the formula (2), is at least 20% in onesurface of the membrane, and said proportion (p) gradually decreasesfrom the one surface to the other surface or that plane within themembrane where A reaches zero, said proportion (p) and a thickness (t)in microns between the one surface and the other surface or a planewithin the membrane in cross-section parallel to the surfaces of themembrane always satisfying the following inequality (b):

    |Δp/Δt|≦12(%/μ)    (b).

In the following Examples and Comparative Examples, the gradients ofproportion (p) to thickness (t) were examined as follows. A cationexchange membrane (r) having the same ion exchange capacity as that of acation exchange membrane (s) to be examined with respect to gradients ofproportion (p) to thickness (t) and containing no pendant sulfonic acidand/or sulfonate groups but pendant carboxylic acid and/or carboxylategroups into which all the --CF₂ SO₂ X groups in the groups of theformula (3) were converted was prepared in substantially the same manneras in the case of the preparation of the cation exchange membrane (s)except that the conversion was completely effected. The membrane (r) wassubjected to measurement of attenuated total reflection spectrum(hereinafter referred to as A.T.R.) to find an absorbance with respectto an absorption band characteristic of carboxylic acid and/orcarboxylate groups, said absorbance being calculated based on the baseline method and being evaluated as 100% in relative absorbance. Themembrane (s) was subjected, on the side of the carboxylic acid and/orcarboxylate group-richer surface, to measurement of A.T.R. to find anabsorbance and a relative absorbance (P.sub. 0 %) with respect to anabsorption band characteristic of carboxylic and/or carboxylate groups.Subsequently, the membrane (s) was shaved on the above-mentioned side bya given thickness (T₁ μ) and subjected to measurement of A.T.R. in thesame manner as described above to find a relative absorbance (P₁ %),said given thickness being obtained by subtracting the thicknessmeasured of the shaved membrane (s) from the thickness measured of theinitial membrane (s). The relative absorbance (P₀ %) corresponds to theproportion (p) in the above-mentioned surface when the thickness (t) is0, and the relative absorbance (P₁ %) corresponds to the proportion (p)in a plane within the membrane in cross-section parallel to the surfacesof the membrane when the thickness (t) is T₁ μ. Substantially the sameprocedures as described above were repeated to find relative absorbances(P₂ %, P₃ %, . . . ) in connection with thicknesses (T₂ μ, T₃ μ, . . . ,respectively). The relative absorbances (P₀ %, P₁ %, P₂ %, P₃ % . . . )were plotted against the thicknesses (0μ, T₁ μ, T₂ μ, T₃ μ . . . ) toform a graph having an ordinate representing the proportion (p) and anabscissa representing the thickness (t). The gradients of proportion (p)to thickness (t) were found from the graph. The measurement of A.T.R.was carried out by using a combination of a diffraction grating infraredspectrophotometer Model IRA-2 and an attenuated total reflector ModelATR-6 (tradenames of products manufactured by Japan SpectroscopicCompany, Ltd., Japan).

The following Examples illustrate the present invention in more detailbut should not be construed as limiting the scope of the invention.

EXAMPLE 1

16 g of ##STR2## 0.16 g of ammonium persulfate and 160 ml of water freeof oxygen were charged into a 500 cc stainless steel autoclave. Theresulting mixture was emulsified with 1.6 g of ammoniumperfluorooctanoate as an emulsifier. The copolymerization was carriedout at 50° C. for 9 hours by introducing tetrafluoroethylene into theemulsion under a tetrafluoroethylene pressure of 5.5 Kg/cm² and using0.007 g of sodium hydrogensulfite as a promotor. Part of the copolymerobtained was saponified with a mixture of 6 N aqueous caustic soda andmethanol (1:1 by volume), and the ion exchange capacity of thesaponified copolymer was examined to find an ion exchange capacity of0.81 milliequivalent per gram of dry polymer.

The rest of the copolymer was washed with water, shaped into a membraneof 250μ in thickness and saponified with a mixture of 6 N aqueouscaustic soda and methanol (1:1 by volume). The resulting membrane wassufficiently dried, and dipped in a solution of phosphorus pentachloridein phosphorus oxychloride (mixing weight ratio=1:3) at 110° C. for 20hours. The membrane thus treated was subjected to measurement of A.T.R.,which showed a strong absorption band at 1420 cm⁻¹ characteristic ofsulfonyl chloride. Between frames made of acrylic resin, two sheets ofthis membrane were fastened in position by means of packings made ofpolytetrafluoroethylene. The frames having the two sheets of themembrane were immersed in a mixed solution of an aqueous 57 weightpercent hydriodic acid solution and glacial acetic acid (volumeratio=11:1; at 72° C. for 17 hours. The membrane was then subjected tosaponification treatment with an aqueous sodium hydroxide solution andto oxidation treatment with an aqueous 5 weight percent sodiumhypochlorite solution at 90° C. for 16 hours. The A.T.R. of the membranewas then measured. The absorption band at 1690 cm⁻¹ characteristic ofcarboxylate (salt of carboxylic acid) groups was observed when themembrane surface treated with the mixed solution containing hydriodicacid was subjected to measurement of A.T.R., whereas the absorption bandat 1060 cm⁻¹ characteristic of sulfonate (salt of sulfonic acid) groupswas observed when the membrane surface not treated with the mixedsolution containing hydriodic acid was subjected to measurement ofA.T.R. The cross-section of the membrane cut perpendicularly to thesurfaces thereof was subjected to dyeing treatment with an aqueousMalachite Green solution having a pH value of 2. A layer having athickness of about 14μ on the side of the membrane surface treated withthe mixed solution containing hydriodic acid was dyed blue while therest of the cross-section of the membrane was dyed yellow [in general, alayer having a proportion (p) of more than about 60% is dyed blue].

The cation exchange membrane obtained in this Example was step-wiseshaved on the side of the membrane surface treated with the mixedsolution containing hydriodic acid and subjected to measurement ofA.T.R. to find a proportion (p) in each step as follows.

    t=0μ . . . p=100%

    t=5μ . . . p=86%

    t=10μ . . . p=72%

    t=15μ . . . p=50%

    t=20μ . . . p=36%

    t=31μ . . . p=0%

From the above data, the graph shown in the Figure was obtained to finda maximum descending gradient (|Δp/Δt|max) of 4.4%/μ.

The performance of the cation exchange membrane of this Example in usefor electrolysis was evaluated as follows. The cation exchange membranewas incorporated in an electrolytic cell having a service area of 0.3dm² (5 cm×6 cm) in such a way that the carboxylate group-containingsurface of the membrane faces the cathode to provide an anode chamberand a cathode chamber separated by the membrane. The anode was a metalelectrode excellent in dimensional stability and the cathode was an ironelectrode. An aqueous saturated sodium chloride solution adjusted to apH value of 3 by continuously adding hydrochloric acid was circulatedthrough the anode chamber, and an aqueous 8 N caustic soda solution wascirculated through the cathode chamber while adding water to thesolution to maintain the caustic soda concentration thereof constant.Under these conditions, electric current was passed between theelectrodes at a current density of 70 A/dm². The electrolysis wascarried out at about 90° C. The current efficiency was calculated bydividing the amount of caustic soda produced in the cathode chamber perhour by the theoretical value calculated from the amount of electricitypassed.

The changes of current efficiency and cell voltage with time are shownin Table 1.

                  TABLE 1                                                         ______________________________________                                        Electrolysis time (hours)                                                                         24     720                                                Current efficiency (%)                                                                            93     93                                                 Cell voltage (volts)                                                                              4.2    4.2                                                ______________________________________                                    

After completion of the electrolysis, the used membrane was visuallyobserved to find no physical damage such as partial peeling-off,cracking, blistering or the like.

COMPARATIVE EXAMPLE 1

A cation exchange membrane was prepared in substantially the same manneras in Example 1 except that an aqueous 57 weight percent hydriodic acidsolution alone was used in place of a mixed solution as used in Example1 and composed of an aqueous 57 weight percent hydriodic acid solutionand glacial acetic acid and the immersion treatment in this hydriodicacid solution was effected at 83° C. for 20 hours. The A.T.R. of themembrane was then measured. The absorption band at 1690 cm⁻¹characteristic of carboxylate (salt of carboxylic acid) groups wasobserved when the membrane surface treated with the hydriodic acidsolution was subjected to measurement of A.T.R. The membrane surfacetreated with the hydriodic acid solution was found to have a proportion(p) of 100%. The cross-section of the membrane cut perpendicularly tothe surfaces thereof was subjected to dyeing treatment with an aqueousMalachite Green solution having a pH value of 2. A layer having athickness of about 6μ on the side of the membrane surface treated withthe hydriodic acid solution was dyed blue while the rest of thecross-section of the membrane was dyed yellow. The maximum descendinggradient (|Δp/Δt|max) of proportion (p) to thickness (t) was 19%/μ.

The performance of the cation exchange membrane of this ComparativeExample in use for electrolysis was evaluated in the same manner as inExample 1.

The changes of current efficiency and cell voltage with time are shownin Table 2.

                  TABLE 2                                                         ______________________________________                                        Electrolysis time (hours)                                                                         24     720                                                Current efficiency (%)                                                                            93     85                                                 Cell voltage (volts)                                                                              4.3    4.6                                                ______________________________________                                    

After completion of the electrolysis, the used membrane was visuallyobserved to find the occurrence of blistering. The cross-section of theused membrane cut perpendicularly to the surfaces thereof was visuallyobserved to find the occurrence of partial peeling-off of thecarboxylate group-containing surface layer portion of about 4μ inthickness.

COMPARATIVE EXAMPLE 2

A copolymer was prepared in substantially the same manner as in Example1 except that the tetrafluoroethylene pressure was 5.0 Kg/cm² instead of5.5 Kg/cm². Part of the copolymer was saponified with a mixture of 6 Naqueous caustic soda and methanol (1:1 by volume), and the ion exchangecapacity of the saponified copolymer was examined to find an ionexchange capacity of 0.83 milliequivalent per gram of dry polymer. Therest of the copolymer was shaped into a membrane (C1) of 50μ inthickness.

On the other hand, 16 g of CF₂ ═CFO(CF₂)₃ COOCH₃, 0.17 g of ammoniumpersulfate and water free of oxygen were charged into a 500 cc stainlesssteel autoclave. The resulting mixture was emulsified with ammoniumperfluorooctanoate as an emulsifier. The copolymerization was carriedout at 50° C. by introducing tetrafluoroethylene into the emulsion undera tetrafluoroethylene pressure of 7 Kg/cm² and using sodiumhydrogensulfite as a promotor. Part of the copolymer was saponified witha mixture of 6 N aqueous caustic soda and methanol (1:1 by volume), andthe ion exchange capacity of the saponified copolymer was examined tofind an ion exchange capacity of 1.10 milliequivalents per gram of drypolymer. The rest of the copolymer was shaped into a membrane (C2) of100μ in thickness.

The membranes (C1) and (C2) were heat-pressed in a state of closecontact with each other to form a laminated membrane which was thensaponified with a mixture of 6 N aqueous caustic soda and methanol (1:1by volume).

The performance of the laminated cation exchange membrane of thisComparative Example in use for electrolysis was evaluated with the outersurface of the membrane (C2) stratum facing the cathode in the samemanner as in Example 1.

The changes of current efficiency and cell voltage with time are shownin Table 3.

                  TABLE 3                                                         ______________________________________                                        Electrolysis time (hours)                                                                         24     720                                                Current efficiency (%)                                                                            92     84                                                 Cell voltage (volts)                                                                              12.4   13.6                                               ______________________________________                                    

After completion of the electrolysis, the used laminated membrane wasvisually observed to find the occurrence of blistering all over thesurface of the membrane. The cross-section of the used laminatedmembrane cut perpendicularly to the surfaces thereof was visuallyobserved to find the occurrence of partial peeling-off at the borderline of lamination of the membranes (C1) and (C2).

COMPARATIVE EXAMPLE 3

A copolymer having an ion exchange capacity due to sulfonate groups of0.71 milliequivalent per gram of dry polymer and an ion exchangecapacity due to carboxylate groups of 1.5 milliequivalents per gram ofdry polymer when saponified with a mixture of 6 N aqueous caustic sodaand methanol (1:1 by volume) was prepared from tetrafluoroethylene,##STR3## and CF₂ ═CFO(CF₂)₄ COOCH₃ according to a customary method knownin the art. The copolymer was shaped into a membrane of 250μ inthickness and saponified with a mixture of 6 N aqueous caustic soda andmethanol (1:1 by volume).

The performance of the cation exchange membrane of this ComparativeExample in use for electrolysis was evaluated in the same manner as inExample 1.

The changes of current efficiency and cell voltage with time are shownin Table 4.

                  TABLE 4                                                         ______________________________________                                        Electrolysis time (hours)                                                                         24     720                                                Current efficiency (%)                                                                            89     82                                                 Cell voltage (volts)                                                                              3.8    3.7                                                ______________________________________                                    

EXAMPLES 2 AND 3

Cation exchange membranes were prepared in substantially the same manneras in Example 1 except that each of the mixed solutions as listed inTable 5 was used in place of a mixed solution as used in Example 1 andthe immersion treatment in each of the mixed solutions was effectedunder treatment conditions as listed in Table 5.

With respect to each cation exchange membrane, the proportion (p) in themembrane surface treated with the mixed solution and the maximumdescending gradient (|Δp/Δt|max) are listed in Table 5 together withthat performance in use for electrolysis which was evaluated in the samemanner as in Example 1.

After completion of the electrolysis, each of the used membranes wasvisually observed to find no physical damage such as partialpeeling-off, cracking, blistering or the like.

                                      TABLE 5                                     __________________________________________________________________________    Example        Treatment                                                                           Proportion(p)                                                                        |Δp/Δt|max                                             Performance*                               No.  Mixed Solution                                                                          Conditions                                                                          in Surface(%)                                                                        (%/μ)                                                                             24 hours**                                                                          720 hours**                          __________________________________________________________________________    2    57 wt % hydriodic acid: propionic acid = 11:1 (by volume)                               72° C. 18 hours                                                              100    4                                                                                     ##STR4##                                                                            ##STR5##                            3    57 wt % hydriodic acid: n-caprylic acid = 500:1 (by weight)                             83° C. 20 hours                                                              100    4.2                                                                                   ##STR6##                                                                            ##STR7##                            __________________________________________________________________________     Note                                                                          ##STR8##                                                                      **electrolysis time                                                      

EXAMPLE 4

A 250μ-thick membrane of a copolymer with pendant sulfonyl fluoridegroups as prepared in Example 1 was treated on one surface thereof witha mixed solution of an aqueous 20 weight percent hydrazine solution andmethanol (volume ratio=1:2) at room temperature for 16 hours. Theresulting membrane was treated with an aqueous 1 N hydrochloric acidsolution at 90° C. for 16 hours, saponified with a mixture of 6 Naqueous caustic soda and methanol (1:1 by volume), and treated with anaqueous 5 weight percent sodium hypochlorite solution at 90° C. for 16hours. The A.T.R. of the cation exchange membrane thus obtained was thenmeasured. The absorption band at 1690 cm⁻¹ characteristic of carboxylate(salt of carboxylic acid) groups was observed when the membrane surfacetreated with the mixed solution containing hydrazine was subjected tomeasurement of A.T.R. The proportion (p) in the membrane surface treatedwith the mixed solution was 80%. The maximum descending gradient(|Δp/Δt| max) was 7%/μ.

EXAMPLE 5

Two kinds of copolymers were prepared in substantially the same manneras in Example 1 except that the tetrafluoroethylene pressures wererespectively 5.0 Kg/cm² and 7.0 Kg/cm². Part of each copolymer wassaponified with a mixture of 6 N aqueous caustic soda and methanol (1:1by volume), and the ion exchange capacity of the saponified copolymerwas examined. The copolymer prepared under a tetrafluoroethylenepressure of 5.0 Kg/cm² and saponified had an ion exchange capacity of0.91 milliequivalent per gram of dry polymer. The copolymer preparedunder a tetrafluoroethylene pressure of 7.0 Kg/cm² and saponified had anion exchange capacity of 0.75 milliequivalent per gram of dry polymer.The rest of the former copolymer was shaped into a membrane (E1) of 100μin thickness, and the rest of the latter copolymer was shaped into amembrane (E2) of 50μ in thickness.

The membranes (E1) and (E2) were heat-pressed in a state of closecontact with each other to form a laminated membrane. The laminatedmembrane was placed on a fabric of polytetrafluoroethylene in such a waythat the outer surface of the membrane (E1) stratum was contacted withthe fabric, and so heated in vacuo that the fabric was embedded in themembrane (E1) stratum for reinforcing the laminated membrane. The fabricof tetrafluoroethylene was an about 0.15 mm-thick leno woven fabrichaving 25 wefts per inch of a 400 denier multifilament and 25 warps perinch of a pair of a 200 denier multifilament.

In the same manner as in Example 1, the reinforced laminated membranewas converted into a membrane containing sulfonyl chloride groups, whichwas then treated on the outer surface of the membrane (E2) stratum witha mixed solution of an aqueous 57 weight percent hydriodic acid solutionand glacial acetic acid (volume ratio=10:1) at 83° C. for 20 hours,followed by saponification with a mixture of 6 N aqueous caustic sodaand methanol (1:1 by volume). The resulting membrane was further treatedwith an aqueous 5 weight percent sodium hypochlorite solution at 90° C.for 16 hours. The cross-section of the thus obtained cation exchangemembrane cut perpendicularly to the surfaces thereof was subjected todyeing treatment with an aqueous Malachite Green solution having a pHvalue of 2. A layer having a thickness of about 11μ on the side of themembrane (E2) surface treated with the mixed solution containinghydriodic acid was dyed blue while the rest of the cross-section of themembrane was dyed yellow. The proportion (p) in the membrane surfacetreated with the mixed solution was 85%. The maximum descending gradient(|Δp/Δt|max) was 4.5%/μ.

The performance of the reinforced laminated cation exchange membrane ofthis Example in use for electrolysis was evaluated in substantially thesame manner as in Example 1 except that an aqueous 4 N caustic sodasolution was circulated through the cathode chamber.

The changes of current efficiency and cell voltage with time are shownin Table 6.

                  TABLE 6                                                         ______________________________________                                        Electrolysis time (hours)                                                                         24     720                                                Current efficiency (%)                                                                            93     93                                                 Cell voltage (volts)                                                                              4.2    4.2                                                ______________________________________                                    

After completion of the electrolysis, the used membrane was visuallyobserved to find no physical damage such as partial peeling-off,cracking, blistering or the like.

EXAMPLES 6 TO 9

From sulfonyl chloride group-containing membranes as prepared in Example5, reinforced laminated cation exchange membranes were prepared insubstantially the same manner as in Example 5 except that each of themixed solutions as listed in Table 7 was used in place of a mixedsolution as used in Example 5 and the immersion treatment in each of themixed solutions was effected under treatment conditions as listed inTable 7.

With respect to each cation exchange membrane, the proportion (p) in themembrane surface treated with the mixed solution and the maximumdescending gradient (|Δp/Δt|max) are listed in Table 7 together withthat performance in use for electrolysis which was evaluated in the samemanner as in Example 5.

After completion of the electrolysis, each of the used membranes wasvisually observed to find no physical damage such as partialpeeling-off, cracking, blistering or the like.

                                      TABLE 7                                     __________________________________________________________________________    Example         Treatment                                                                           Proportion(p)                                                                        |Δp/Δt|max                                             Performance*                              No.  Mixed Solution                                                                           Conditions                                                                          in Surface(%)                                                                        (%/μ)                                                                             24 hours**                                                                          720 hours**                         __________________________________________________________________________    6    57 wt % hydriodic acid: glacial acetic acid = 2:1 (by                                    60° C. 17 hours                                                              60     4.3                                                                                   ##STR9##                                                                            ##STR10##                          7    57 wt % hydriodic acid: propionic acid = 3:1 (by volume)                                 60° C. 17 hours                                                              40     5.0                                                                                   ##STR11##                                                                           ##STR12##                          8    47 wt % hydrobromic acid: glacial acetic acid = 3:1 (by                                  90° C. 16 hours                                                              65     3.5                                                                                   ##STR13##                                                                            #STR14##                          9    30 wt % hypo- phosphorous acid: n-caprylic acid = 600:1 (by                              90° C. 16 hours                                                              53     4.0                                                                                   ##STR15##                                                                           ##STR16##                          __________________________________________________________________________     Note                                                                          ##STR17##                                                                     **electrolysis time                                                      

EXAMPLE 10

A reinforced laminated cation exchange membrane was prepared insubstantially the same manner as in Example 6 except that a mixedsolution of an aqueous 57 weight percent hydriodic acid, glacial aceticacid and n-caprylic acid (volume ratio=2:1:0.003) was used in place of amixed solution as used in Example 6. The proportion (p) in the membranesurface treated with the mixed solution was 61%, and the maximumdescending gradient (|Δp/Δt|max) was 4.2%/μ.

What is claimed is:
 1. In a process for the preparation of a fluorinatedcation exchange membrane used for electrolysis comprising treating, witha treating agent selected from the group consisting of an aqueousreducing solution of an inorganic acid, an aqueous reducing solution ofan inorganic salt, an aqueous reducing solution of hydrazine, an aqueousreducing solution of an inorganic acid and an inorganic salt, and anaqueous reducing solution of an inorganic salt and hydrazine, onesurface of a membrane comprising a fluorocarbon polymer containingpendant groups of the formula (3):

    --OCF.sub.2 CF.sub.2 SO.sub.2 X                            (3)

wherein each X independently is fluorine, chlorine, bromine, hydrogen,ammonium, a quaternary ammonium or a metal atom,to convert part of thependant groups of the formula (3) into pendant groups of the formula(1):

    --OCF.sub.2 COOM                                           (1)

wherein each M independently is hydrogen, ammonium, a quaternaryammonium or a metal atom,said fluorinated cation exchange membranecomprising a fluorocarbon polymer containing pendant carboxylic acidand/or carboxylate groups and pendant sulfonic acid and/or sulfonategroups, the proportion of the density of pendant carboxylic acid and/orcarboxylate groups relative to the total density of pendant carboxylicacid and/or carboxylate groups and pendant sulfonic acid and/orsulfonate groups being different between one surface and an internalplane in cross-section parallel to the surfaces of the membrane; theimprovement wherein one surface of a membrane comprising a fluorocarbonpolymer containing pendant groups of the formula (3):

    --OCF.sub.2 CF.sub.2 SO.sub.2 X                            (3)

wherein each X independently is fluorine, chlorine, bromine, hydrogen,ammonium, a quaternary ammonium or a metal atom, is treated with atreating agent selected from the group consisting of an aqueous reducingsolution of at least one inorganic acid, an aqueous reducing solution ofat least one inorganic salt, an aqueous reducing solution of hydrazine,an aqueous reducing solution of at least one inorganic acid and at leastone inorganic salt, and an aqueous reducing solution of at least oneinorganic salt and hydrazine, in the presence of at least one reactioncontrolling agent selected from the group consisting of C₁ -C₁₂carboxylic acids, C₁ -C₁₂ sulfonic acids, C₁ -C₁₂ alcohols, C₁ -C₁₂nitriles and C₂ -C₁₂ ethers, provided that when the treating agentincludes hydrazine, said at least one reaction controlling agent isselected from C₁ -C₁₂ alcohols, C₁ -C₁₂ nitriles and C₂ -C₁₂ ethers,thereby to prepare a fluorinated cation exchange membrane whichcomprises a fluorocarbon polymer containing pendant groups of theformula (1):

    --OCF.sub.2 COOM                                           (1)

wherein each M independently is hydrogen, ammonium, a quaternaryammonium or a metal atom, and pendant groups of the formula (2):

    --OCF.sub.2 CF.sub.2 SO.sub.3 M                            (2)

wherein each M independently is as defined above, and wherein theproportion (p) represented by the equation (a):

    p=A/(A+B)×100(%)                                     (a)

wherein A is the density of pendant groups of the formula (1) and B isthe density of pendant groups of the formula (2), is at least 20% in onesurface of the membrane, and said proportion (p) gradually decreasesfrom the one surface to the other surface or to that plane within themembrane where A reaches zero, said proportion (p) and a thickness (t)in microns between the one surface and the other surface or a planewithin the membrane in cross-section parallel to the surfaces of themembrane satisfying the following inequality (b):

    |Δp/Δt|=12(%/μ)           (b).


2. A process according to claim 1, wherein said treating agent and saidat least one reaction controlling agent are in the form of a mixturethereof.
 3. A process according to claim 1 or 2, wherein X is chlorine.4. A process according to claim 3, wherein said membrane to be treatedis made of a composite membrane composed of two kinds of films offluorocarbon polymers differing in equivalent weight by 150 or more, andwherein the outer surface of the film of the fluorocarbon polymer with ahigher equivalent weight is treated as recited therein.